1. Field of the Invention
This invention relates to an object locating method and more particularly pertains to a passive method for detecting the range and azimuth of active and passive targets.
Typically, in an electronic warfare scenario, the enemy may employ a high-power search radar having a regular circular scan. When the radar detects a penetrator, such as an aircraft, the radar vectors another aircraft to intercept the penetrator. Thus, it is very desirable for the penetrator to locate in range and azimuth both the enemy radar (an active target) and the enemy interceptor (a passive target) passively, that is, without using the penetrator's radar.
2. Description of the Related Art
One basic problem in electronic warfare has been to locate in range and azimuth an enemy emitter, such as radar, without actively radiating an RF signal which can betray the radiating position. In the past the azimuth part of this problem has been solved with various types of direction finding (DF) equipments which are typically included in military platforms such as aircraft and ships. Passive ranging however is a much more difficult problem and has just now started to yield to effective solution.
A first solution is to use the DF equipment to achieve successive DF angles. For example, an aircraft 10, which carries DF equipment, flies a baseline distance 12 as shown in FIG. 1. By triangulation the range to an emitter 14 may be found as energy from the emitter is received by the aircraft 10. This technique works best broadside to the aircraft 10 where a rather large triangle baseline 12 may be effected. A problem with this approach is the relatively long time required to fly the baseline. Another problem is that this approach works very poorly directly off the nose of the aircraft because a much smaller baseline and, therefore, a smaller triangle is effected. This is unfortunate since the most interesting signals are in the nose aspect of the aircraft, i.e., from emitters that the aircraft is approaching, and the aircraft will very shortly overfly these emitters.
A second technique is to estimate the range by means of power received. This technique relies upon the fact that in free space power received is inversely proportional to range squared. The problem with this technique is that multipath effects, over the horizon attenuation effects, and power level quantizing effects lead to very large range estimation errors.
A third approach, depicted in FIG. 2, is known as PROSE (Passive Ranging On Scanning Emitters), also known as LRPLS (Long Range Passive Location System). This technique relies upon the exploitation of the scan rate of a circular scan search radar 16. In the implementation of this technique two antennas 18 and 20 are placed on an aircraft as far apart as possible, such as on wing tips. The two antennas 18 and 20 intercept the radar beam at slightly different times as the beam swings by the aircraft position. The intercepted blip envelope from the second antenna overlaps the blip envelope from the first antenna but is slightly shifted in position one from the other. By careful processing in a computer the centroid of one blip may be separated from the other and the time differential (TD) when the center of the radar beam passed each antenna may be determined. This calculation yields the angle subtended, .OMEGA., at the radar 16 by the two wing tip antennas 18 and 20 as follows ##EQU1##
A DF system on the aircraft records independently the angle of incoming radar signals. The triangle baseline 22 is known (wing tip to wing tip) and thus the triangle 22, 24, 26 may be solved.
The problem with this technique is that the triangle 22, 24, 26 is a very long one with a very short baseline 22. The accuracy depends heavily upon how accurately the time between the two blip centroids can be determined. In order to get a reasonable 5 percent to 10 percent range accuracy the computer must average as many as 10 blips plus all side and back lobes. This process usually takes as long as 100 seconds to effect one range measurement.
A most significant problem with all three of the above techniques is that these techniques detect the location of only the emitter, an active target, but not the location of a passive target.
A fourth technique named APLS (Airborne Passive Location System) is related to LRPLS and PROSE in that APLS is a bistatic technique which first uses PROSE to determine one leg of a larger triangle. Once this is determined then bistatic reflections from passive targets may be intercepted and their locations derived by triangulation. APLS suffers from the problem of first requiring the range of the radar to aircraft to be determined by PROSE before APLS can detect and range on other passive targets.
All the above techniques suffer from relatively long integration periods necessary to achieve an output range. PROSE type solutions suffer from the need for special wing tip antennas which are typically large low frequency (3 GHZ) types. For example, on some aircraft it is inconvenient to locate such antennas on the wing tips because of an air drag problem, because of the problem of routing cables or waveguide through the wing, and past a pivot point in pivoting wing aircraft, and because of the priority that other ECM functions have upon such wing tip antenna locations.
It is because of these deficiencies that the Bistatic Object Location Method was developed. The present invention utilizes a DF system, including antennas, existing on an aircraft or any other platform, such as a ship, satellite, land vehicle, or fixed station. The invention provides a high degree of accuracy from a very wide triangle, effectively increasing the baseline of prior art techniques measured in feet to a baseline measured in miles. No wing tip antennas are required because the invention uses only fuselage-mounted DF antennas. Only 1 blip width (about 30 milliseconds) is required, following synchronization to the radar scan period, by this method to produce range and direction of both a radar and passive target reflectors. The invention can operate on a multitude of passive target reflectors within the preview of a radar and the platform using the method during every radar scan period. The present invention detects the location of not only a radar but passive target reflectors, such as interceptor aircraft, as well. Thus, the invention provides fast, accurate range and azimuth location information on both an active (radar) target and a multitude of passive targets. The present invention can be used to locate additional active targets sequentially by appropriately adjusting the operating frequency of the DF system.